1. Field of the Invention
The present invention relates to a vehicle suspension system for a steerable drive wheel and in particular, to a vehicle suspension system for a steerable drive wheel which makes it easy to set up an imaginary kingpin axis, improves handling safety by setting up an imaginary link and minimizing the variation ratio of the height of a roll center, enhances free layout degree with respect to a change in the angle of camber and tread, and especially ensures maximum effective volume in an engine room by minimizing the space occupied by a vehicle suspension system.
2. Description of Related Art
In general, a vehicle suspension system for a steerable drive wheel connects an axle shaft and a vehicle body to each other and absorbs vibrations and impacts from a road surface while running so that safety of a vehicle, and an improved riding comfort can be obtained. To absorb impacts, a vehicle suspension system should be flexibly connected vertically, and to ensure driving force, braking force and centrifugal force during a vehicle's turning, which are generated at the wheel, a vehicle's suspension should be horizontally connected firmly. The vehicle suspension system is classified structurally as a suspension system integrated with an axle and an independent suspension system. While a suspension system integrated with an axle is applied to large vehicles like buses, trucks and rear axles of passenger cars, an independent suspension system is applied mainly to an axle of a passenger car, to improve ride comfort and running safety by bisecting an axle and allowing both wheels to act independently. Among these suspension systems, the present invention relates to an independent suspension system.
There are many kinds of independent suspension systems. As a suspension system applied to a steerable wheel, Macpherson type and Wishbone type are applied widely.
A vehicle suspension system for a steerable wheel is designed to control a position of a wheel with respect to the vehicle body to obtain a better positioning in a vehicle operation as well as absorb vibrations or impact. The axle is thereby prevented from directly transmitting impact or vibration from a road surface to the vehicle body, so that there can be obtained an optimal handling safety and running safety.
The front wheel is mounted to be changeable in its direction from right to left or from left to right, centering the kingpin, and is also mounted with a geometry angle to meet requirements for the front suspension.
Mounting the front wheel with the geometry angle is called wheel alignment. The wheel alignment is determined by several elements, but the optimal operation of the front wheel is, however, achieved by the supplemental operation of the elements with respect to each other. A kingpin inclination among the elements reduces a handling force of a steering wheel with a camber, absorbs any impact generated from the vehicle motion and then increases a restitution force of the steering wheel. The turning safety of the straight ahead position of the vehicle and the handling safety in a vehicle's turning position are thereby obtained.
The kingpin inclination is such that the kingpin is mounted with its central line lying at a predetermined angle with respect to the vertical line when the vehicle is viewed from the front. An interval between the central line of the kingpin and a central line of the wheel is called offset. The offset is generally characterized as an offset at the wheel center and an offset at the ground. The offset at the wheel center has an effect on the straight ahead characteristics of the vehicle when a driving force and an engine brake are applied to the wheel. The offset at the ground has an effect on the handling safety when the vehicle brakes and turns. If the offset at the ground is decreased, the sensitivity with respect to the steering is decreased, in the same manner as the handling safety is increased when the vehicle brakes and turns.
Vibration has an effect on the vehicle safety, including rolling, pitching, yawing as well as bouncing of a vehicle during driving. If these vibrations are absorbed softly, not only ride comfort but also stability is improved.
As the vibration of a vehicle is concerned, a vibration generated when a vehicle is turning is defined as rolling, which means that a vehicle vibrates in both side directions, and rolling has an important effect upon turning safety. A certain basic point where rolling occurs is called a roll center. Rolling is certain to occur because the centroid of a vehicle is higher than a roll center. Accordingly the more the height of a roll center changes, the greater the gravity of a vehicle moves. Therefore this phenomenon results in a greater slip angle so that running safety and handling safety become worse. Accordingly, to ensure running safety, the variation ratio of the height of roll center is desired to be minimized.
Considering the above, the prior suspension system is described in detail hereinafter.
FIG. 3A illustrates a general Macpherson type suspension system including a strut arm 73 formed integrally with a steering knuckle 71, provided elastically with a spring 74 at an upper end and contains a shock absorber 72, and a lower arm 75 that is attached to the steering knuckle 71 by a ball joint 76 and to the vehicle body by a hinge.
This suspension system has some advantages of simple structure, a small spring, light mass, and a small effective volume of an engine room. But in order to make the kingpin offset(a) reduced or minus, the upper supporting point 77 of the strut arm 73 is to be moved toward the engine room, or the lower end of the strut arm 73 connected to the ball joint 76 is to be moved outside of the engine room. However, if the upper supporting point 77 of the strut arm 73 is moved toward the engine room, the effective volume of an engine room is reduced and the kingpin angle(.alpha.) becomes excessively large only to have a bad effect on a vehicle's ability to turn. If the lower end of the strut arm 73 connected to the ball joint 76 is moved outside, it is actually impossible to reduce the kingpin offset(a) because it will interfere with a brake disk attached to a wheel.
FIG. 3B illustrates a general Wishbone type suspension system including upper and lower control arms 81, 82, steering knuckle 83, a spring assembly 85 including a shock absorber 84 and ball joints 86, 87 connecting the upper and lower control arms 81, 82 to the steering knuckle 83. In order to adjust the kingpin offset(a) the vehicle body side connecting portion 88 of the upper control arm 81 is to be moved toward the engine room or the ball joint 87 of the lower control arm 82 is to be moved outside. But, in such a Wishbone type suspension system, like the Macpherson type suspension system, if the vehicle body side connecting portion 88 of the upper control arm 81 is moved toward the engine room, the effective volume of an engine room is reduced and the kingpin angle(.alpha.) becomes excessively large to adversely affect turning ability of a vehicle, and if the ball joint 87 of the lower control arm 82 is moved outside, it is actually impossible to reduce the kingpin offset because it will interfere with a brake disk attached to the wheel.